1. Field of the Invention
The present invention relates to a process for making silica powder by a sol-gel process wherein an aqueous suspension of a silica-based raw material is made, said suspension is gelled, the gel obtained is dried and the dried gel is fractionated into powder made up of silica granules. It also relates to the use of such powder to manufacture an optical fiber preform.
2. Description of the Prior Art
One process for making silica powder is disclosed in French patent 2 693 451. In the examples given therein, a suspension is made from particles essentially free of chlorine. Silica particles are made by oxidation of silicon chloride SiCl4, generally in the presence of hydrogen and oxygen. The particles obtained directly by such a process are referred to as xe2x80x9cacidxe2x80x9d because they contain chlorine in the form of hydrochloric acid. Such particles are corrosive. The particles then generally undergo a chlorine desorption process yielding xe2x80x9cneutralizedxe2x80x9d particles. The xe2x80x9cneutralizedxe2x80x9d particles are still acid particles as, in the case of a 4 wt. % aqueous dispersion of particles having a specific surface area equal to approximately 50 m2/g, their pH is estimated at from 3.8 to 4.8, for example. However, they are much less acid than before the chlorine desorption treatment. The cost of manufacturing such silica particles is therefore high. One example of xe2x80x9cneutralizedxe2x80x9d particles is Aerosil OX 50 silica soot from DEGUSSA, consisting mainly of particles with a specific surface area of 50 m2/g.
The object of the process according to the invention is partly or totally to avoid the use of such essentially chlorine-free particles.
To this end, the process according to the invention is a process for making silica powder by a sol-gel process wherein an aqueous suspension of a silica-based raw material which includes particles containing chlorine is made, the suspension is gelled, the gel obtained is dried and the dried gel is fractionated into powder made up of silica granules.
Surprisingly, particles containing chlorine can also be used to make silica powder suitable for manufacturing optical fiber preforms.
Because they have not undergone any chlorine desorption treatment, particles containing chlorine have the advantage that their cost is very much lower than particles which are essentially free of chlorine.
In one embodiment of the invention, the process according to the invention includes screening the silica granules obtained by fractionation to obtain powder made up of granules having sizes from a lower limit from 75 xcexcm to 120 xcexcm to an upper limit from 280 xcexcm to 550 xcexcm. The size of a granule is defined by the greatest length of the granule, regardless of its shape. It is principally measured by screening.
The silica particles containing chlorine used in the process according to the invention generally have a specific surface area not greater than 80 m2/g. In the case of screening, using starting particles of the above kind has the advantage of leading to a good process yield in terms of granules. The yield in terms of granules is defined as the percentage by weight of granules after screening to the proportion of silica contained in the raw material in suspension.
In a preferred embodiment of the invention the silica-based raw material includes particles containing chlorine having a specific surface area less than or equal to 80 m2/g and less than 5 wt. %, preferably from 0.5 wt. % to 1.5 wt. %, relative to the total quantity of said particles containing chlorine of silica particles having a specific surface area from 150 m2/g to 300 m2/g. Surprisingly, this increases the viscosity of the suspension and the yield in terms of granules, which generally have an approximate size from 100 xcexcm to 500 xcexcm or even from 100 xcexcm to 300 xcexcm. In particular, screening is improved because there is advantageously little clogging of the screen, essentially because of the reduced content of granules with a size less than about 100 xcexcm, which can tend to clump together.
The silica particles with a specific surface area from 150 m2/g to 300 m2/g preferably contain chlorine. However, such particles can instead be essentially free of chlorine because their concentration in the suspension is low and so the cost of the process according to the invention remains highly advantageous.
In another embodiment of the process according to the invention, in which screening is performed as previously described, the raw material based on silica further includes silica granules having a size greater than the upper screening limit. There are therefore at least two silica granule fabrication steps as previously defined, the last fabrication step being such that the silica-based raw material additionally includes silica granules rejected by screening in at least one of the preceding fabrication steps. As described in U.S. patent application Ser. No. 09/116,337, it is possible in this way to recycle non-standard silica granules (for example 100 xcexcm to 500 xcexcm granules) rejected by screening by adding them to the silica-based raw material. In this case it is generally preferable to add to the initial suspension the non-standard granules fraction with a size greater than the upper screening limit, which is 500 xcexcm, for example, but which can instead be 300 xcexcm for screening from about 100 xcexcm to about 300 xcexcm, for example. Surprisingly, this embodiment produces a suspension of adequate viscosity, which advantageously leads to an improved screening yield in terms of granules, which generally have an approximate size from 100 xcexcm to 500 xcexcm or even from 100 xcexcm to 300 xcexcm.
In one embodiment of the process according to the invention the pH of the suspension is adjusted to a value between 3 and 4.5. This advantageously limits corrosion of the reactor in which the suspension is prepared.
The dry gel is fractionated into silica granules generally having a diameter from a few xcexcm to 2 mm, as is well known to the skilled person. The granules generally have an apparent density in the order 0.5 g/cm3 to 0.6 g/cm3. On an industrial scale, such fractionation is generally effected by means of a screw-type granulator.
The acid particles are generally taken up into solution in an amount from 30 wt. % to 76 wt. %, for example equal to 70 wt. %.
The invention also relates to using a powder made by the process previously described to manufacture an optical fiber preform. The granulated powder obtained by the process according to the invention can be used directly or after densification in a plasma torch during plasma surfacing of a primary preform.
In a preferred embodiment of the invention, the powder made by the process described above is screened before it is used to manufacture an optical fiber preform. The screened powder is then generally made up of granules with a size ranging from a lower limit from 75 xcexcm to 120 xcexcm to an upper limit from 280 xcexcm to 550 xcexcm.
As described in French patent 2 693 451, such surfacing can be carried out in the presence of a fluorine-containing gas and it is also possible to add at least one dopant to the silica granules obtained in this way.
As described in French patent 2 693 451 it is also possible to apply a densifying heat treatment to the silica granules and to eliminate some of the OH groups by an intermediate phase of the heat treatment.